Pretreating metal substrates with a phosphate conversion coating and rinsing with a chrome-containing sealer is well known for promoting corrosion resistance and improving the adhesion of subsequently applied decorative and protective coatings. Cationic electrodeposition compositions are typically applied over phosphated steel substrates to further improve corrosion resistance. While the combination of phosphate conversion coating and electrodeposited coating provides superior corrosion resistance, heavy metals typically used in such coatings can provide environmental disposal concerns. For example, phosphate conversion coating compositions typically contain heavy metals such as nickel and post-rinses contain chrome, while cationic electrodeposition compositions typically are formulated with lead as a pigment or soluble lead salt. Also, conventional phosphating processes can require eleven to twenty-five stages that occupy a large amount of physical space in a plant and require significant capital investment. Another drawback of conventional phosphating processes is difficulty in coating mixed-metal objects including aluminum.
Nickel-free phosphate coating compositions and chrome-free rinsing compositions having comparable corrosion resistance to nickel- and chrome-containing compositions are highly desirable. Rinsing compositions utilizing metal ions other than chromium are disclosed, for example, in U.S. Pat. Nos. 3,966,502 and 4,132,572. U.S. Pat. No. 3,966,502 discloses treatment of phosphated metals with zirconium-containing rinse solutions. U.S. Pat. Nos. 3,912,548, 5,209,788, and 5,653,823 disclose other rinse compositions containing combinations of Group IVB metal ions with polymeric materials that have been used over phosphated substrates. However, many post-rinse compositions are suitable for use over a limited number of substrates or over substrates that must be phosphated first.
It would be desirable to provide a simplified pretreatment process free of heavy metals for coating metal substrates, including mixed metal substrates such as are commonly found on today's automobile bodies. Such a pretreatment process, when combined with a lead-free electrodeposition process, would provide an environmentally friendly alternative for providing corrosion resistance to metal substrates.